1. Field of the Invention
This invention relates to structural improvements for acoustic wave devices.
2. Description of the Prior Art
The interdigital type of electromechanical transducer is used frequently in surface acoustic wave devices. Such a transducer typically comprises a pair of electrodes formed on an outer surface of a body of piezoelectric material. Each electrode has a plurality of spaced-apart fingers which extend toward the other electrode across the path for propagation of the surface acoustic waves. The fingers of one electrode are typically interdigitated or alternated with the fingers of the other electrode along the surface acoustic wave propagation path.
The spacing between the fingers of each electrode of the transducer typically has a uniform or constant periodicity. In addition, the periodicity of the spacing of the two sets of fingers is the same. If this were not so, there would be a tendency for fingers of one electrode to contact fingers of the other electrode, creating a short circuit between the two electrodes. In the usual case, each electrode finger is positioned to be equidistant from its two nearest neighbor fingers of the other electrode. It may be said that this symmetrical positioning provides a constant phase displacement between the two sets of fingers of 180 degrees. In this case, where the electrode fingers are symmetrically positioned, the coupling between the electrical fields and the acoustic waves becomes synchronous and therefore very strong when the signal frequency is such that the center-to-center spacing between adjacent electrode fingers is one-half of an acoustic wavelength.
It is very desirable that an acoustic wave device have low insertion loss. High insertion loss is disadvantageous because it is accompanied by a requirement for correspondingly high power consumption in the signal driver and signal detector for the device. However, the surface acoustic wave devices using interdigital transducers which have been heretofore available have had insertion losses higher than is desirable. When ways are found to make these insertion losses lower, as in the invention disclosed hereinafter, many additional applications will be found for these devices.
In the typical embodiments of the prior art, surface acoustic wave devices having interdigital transducers formed on a surface of a substrate of piezoelectric material are operated in air. The transducer is therefore exposed to air on one side. This operation in air is one of the reasons for the occurrence of relatively high insertion loss. This is because the air surrounding the outer half of the transducer is not piezoelectrically active and therefore does not contribute to the coupling between electrical and mechanical activities. Stated alternatively, since part of the electrical energy stored in the capacitor formed by the transducer electrodes is stored in air, that part of the electrical energy is not involved in and does not contribute to electromechanical coupling. It follows that the efficiency of the associated electrical-mechanical energy conversion is correspondingly low. In addition, the power handling capacity for a surface acoustic wave device operated in air is relatively low and limited because the largest electric field intensity which can be induced without causing dielectric breakdown and arcing in the air is relatively low.
The prior art interdigital transducers described above are bidirectional. When these transducers are appropriately energized by electrical signals, redundant surface acoustic waves are excited to propagate in both opposing directions along the propagation path on the piezoelectric material. The propagation path for the waves extends in a direction perpendicular to the transducer electrode fingers. In the typical situation, only one of the two counter-propagating waves thus excited is of interest. The energy of the other wave is expended and wasted in an acoustic energy absorber. This energy waste is another cause of the undesirably high insertion loss of prior-art acoustic wave devices.
It is apparent that the high energy loss due to the bidirectionality of the typical interdigital transducer could be reduced or even eliminated if the transducer were made to launch surface acoustic waves unidirectionally. It is known that wide-band unidirectionality can be achieved, in theory, by using, for example, three or four sets of electrode fingers driven by an appropriate multiple-phase alternating signal. Attempts have been made to fabricate multiple-phase unidirectional interdigital transducers by designing each electrode finger to be much narrower or thinner than is otherwise customary. The neighboring transducer fingers can then be positioned closer to each other than one-half of an acoustic wavelength. The phase displacement between neighboring transducer fingers is then less than 180 degrees. More than two of such fingers can be disposed per acoustic wavelength. When this is done, three or four electrodes, for example, can be disposed for synchronous multiple-phase driving.
However, it has proved to be difficult to implement such a multiple-phase surface acoustic wave transducer where the three or four electrodes are disposed primarily on a single surface. In order to avoid short circuits between overlapping parts of different electrodes, portions of the fingers of at least one electrode must be elevated over at least one other electrode. Such a transducer is expensive to fabricate. In addition, the unsupported structure of the elevated portion of the transducer is somewhat delicate and the fabrication process is therefore likely to have a low yield.
Regardless of considerations of directionality, it is sometimes desirable to propagate surface acoustic waves on a substrate of conductor or semiconductor material. However, when surface acoustic wave transducer electrodes are deposited directly on such a substrate, the substrate will tend to short-circuit the electrodes and thus impair the operation of the transducer. Even in cases where the surface of the conductor or semiconductor substrate has been oxidized to provide a layer of dielectric material thereon, that layer is typically very thin. Thus there remains strong capacitive coupling between the transducer electrodes and the substrate. This coupling keeps the loading on the transducer high and the electromechanical energy conversion efficiency low.